This invention relates to a control system for controlling the fuel supply to a combustion engine having two or more sets of burners.
In staged combustion gas turbine engines, the arrangement of the burners commonly includes a set of pilot burners, which are on at all times when the engine is running, a set of idling burners, providing a steady low speed operation when the engine is at idle, and one or more sets of main burners, which are fired in stages as thrust demand increases. Ideally, the control system for controlling the supply of fuel to the burners should have the ability to maintain the rate of fuel flow to each set of burners, even when the flow to other sets is changing. Problems can arise with single pump control systems as a reduction in flow can occur in the fuel supply to the pilot and idling burners when a set of main burners is switched on.
In staged combustion gas turbine engines, it is also a requirement to be able to purge the part of the fuel supply path to a set of burners exposed to high ambient temperatures when the burners are switched off, otherwise the paths are liable to block as a result of solid carbon particles forming in the residual fuel. Furthermore, once a path has been purged it is desirable to minimise the delay that occurs in refilling the path when the burners are next switched on. It is also desirable that, since failure of the fuel supply will cause the engine to shut down with potentially dangerous consequences, for example on aircraft, the system should contain some form of back up or redundancy.
It is an object of the invention to provide a fuel supply control system in which at least one of these requirements is met or one of the aforementioned problems is alleviated.
According to a first aspect of the present invention there is provided a control system for use in controlling fuel supply to at least two sets of burners of an engine, the control system comprising two or more fuel pumps, each of the pumps being driven, in use, by a respective variable-speed motor, and control means for controlling the speed of the motors so as to regulate the rate of flow of fuel to each of the burners.
Preferably, each of the fuel pumps is driven by means of a variable-speed electric motor.
In one embodiment, the control system may comprise two pumps, each of the pumps having an inlet and an outlet,
a first fuel flow path through which fuel flows, in use, from the outlet of a first one of the pumps to a set of pilot burners and to a set of idling burners, and
a further fuel flow path through which fuel flows from the outlet of the other of the pumps to a set of further burners,
the set of idling burners and each set of further burners each preferably having, associated therewith, respective staging valve arrangements.
The staging valve arrangements may be electrically operated valves. Alternatively, pressure operated check valves may be used.
The control system may further comprise means for controlling operation of the staging valve arrangements.
Preferably, the set of further burners comprises a first set of main burners and a second set of main burners. The control means may take the form of an electronic controller associated with the aircraft engine.
It is an advantage of the invention that the flow of fuel through the pumps may be rapidly increased to a higher level for a short period of time (i.e. xe2x80x9cspikedxe2x80x9d), in order to substantially maintain the rate of flow of fuel to one or more sets of burners when another or others of the sets of burners are switched on (i.e. during priming of the newly switched on burners). Additionally, the control algorithms are simpler if each fuel flow path is supplied with fuel from a separate electrically driven pump.
The first fuel flow path may be provided with a first flow sensing valve for monitoring the rate of fuel flow through the first fuel flow path, whereby the first flow sensing valve provides a first output signal indicative of the rate of flow of fuel through the first fuel flow path, the control means receiving the first output signal such that the speed of at least one of the motors is controlled in response to the first output signal.
The further fuel flow path may be provided with a further flow sensing valve for monitoring the rate of fuel flow through the further fuel flow path, whereby the further flow sensing valve provides a further output signal indicative of the rate of flow of fuel through the further fuel flow path, the control means receiving the further output signal such that the speed of at least one of the motors is controlled in response to the further output signal.
Conveniently, at least one of the first or further flow sensing valves may be provided with a position sensor for monitoring the position of the associated flow sensing valve, the position sensor providing an output signal indicative of the rate of flow of fuel through the respective fuel flow path.
A linear variable differential transducer (LVDT) may be used as the position sensor.
In one embodiment of the invention, the further fuel flow path may be provided with a split valve arrangement for dividing the fuel flow into first and second staging fuel flow paths. In use, fuel flows through the split valve arrangement into the first staging fuel flow path to a first set of main burners and into the second staging fuel flow path to a second set of main burners, the proportion of the total flow of fuel into the split valve that flows to the first set of main burners being controlled by controlling the position of the split valve arrangement.
Each staging fuel flow path may be provided with a flow sensing valve for monitoring the rate of fuel flow through the respective staging fuel flow path, whereby the flow sensing valve associated with each staging fuel flow path provides an output signal indicative of the rate of flow of fuel through the associated staging fuel flow path.
In any of the embodiments herein described, the fuel flow path delivering fuel to the set of pilot burners may be provided with a pressure raising shut-off valve (PRSOV), which serves to maintain a fuel pressure within the system above a predetermined value and prevents the flow of fuel to the set of pilot burners if the fuel pressure falls below a predetermined value.
Conveniently, the outlets of the pumps may be connected to each other to form a common supply for each set of burners. For example, for an engine having three sets of burners, the common supply may deliver fuel to first, second and third fuel flow paths associated with the first, second and third sets of burners respectively. Each of the first, second and third flow paths may be provided with a flow regulating valve and a flow sensing valve, the control means including a means of controlling the flow regulating valves in response to an output signal provided by the respective flow sensing valve.
It is an advantage of this embodiment of the invention that failure of one of the pumps will not give rise to a complete loss of fuel supply to all of the burners and will permit continued operation of the engine.
Each flow regulating valve may include an inlet port and an outlet port, the flow regulating valve being movable to an open position in which fuel at high pressure is able to flow through the associated fuel flow path to the associated set of burners.
Each flow regulating valve may further include a low pressure inlet port and a low pressure outlet port, the flow regulating valve being movable to a closed position in which high pressure fuel flow is prevented but fuel at low pressure is able to flow through the low pressure inlet port, through the low pressure outlet port and into the associated fuel flow path to permit priming of the associated set of fuel manifolds with low pressure fuel.
Each main manifold may have an associated air vent valve arrangement, the air vent valve arrangement being opened during priming of the respective main manifold in order that air and/or other gases in the respective fuel flow path is able to escape through the air vent valve arrangement. Any low pressure fuel which may escape through the air vent valve arrangement flows to a low pressure drain. On completion of the priming of a set of burners, the air vent valve arrangement is closed, the respective flow regulating valve remains closed and flow of the low pressure fuel to the main burners is prevented by the staging valves which remain closed.
Conveniently, the PRSOV associated with the pilot burners, may be used to purge the manifolds and fuel supply paths between the flow regulating valves and the associated burners on shut down by permitting fuel in the fuel flow path through which fuel flows to the associated set of burners to flow through the PRSOV to the low pressure drain after the burners have been switched off.
According to a second aspect of the invention, a method of controlling fuel supply to at least two sets of burners comprises the steps of providing two or more fuel pumps, driving each of the fuel pumps by a respective variable-speed motor, and controlling the speed of the motors so as to regulate the rate of flow of fuel to each of the sets of burners.
It will be appreciated that the preferred and/or optional features of the first aspect of the invention may be used to carry out preferred and/or optional steps of the second aspect of the invention.